Kd is ratio of
The equilibrium dissociation constant Kd is the ratio of the dissociation rate constant to the association rate constant. A higher numerical value of Kd therefore indicates that dissociation is favored relative to association, resulting in weaker net binding. Affinity is consequently the reciprocal of Kd. This inverse relationship is fundamental to the quantitative interpretation of all reversible ligand–receptor interactions.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)